1. Field of the Invention
This invention relates to a method of making a catalytic combustion type gas detector which utilizes the catalytic action of a sintered, high-melting metal oxide such as TiO.sub.2, ZrO.sub.2, HfO.sub.2, Y.sub.2 O.sub.3, CeO.sub.2, NiO, Cr.sub.2 O.sub.3 or equivalent and which has selectively decreased sensitivity to alcohol vapors and to tobacco fumes.
2. Description of the Prior Art
Catalytic combustion type gas detectors heretofore employed were generally utilizing as catalyst a metal of the platinum group and fixed on a carrier such as alumina or the like. These conventional gas detectors have the defects of being highly susceptible to catalytic poisoning and to moisture, resulting in a marked degradation of the catalytic action, in a high concentration of flammable gas and in high manufacturing costs. The present inventors discovered that the abovesaid defects could be overcome by exploiting the catalytic action of a sintered, high-melting metal oxide such as TiO.sub.2, ZrO.sub.2, Cr.sub.2 O.sub.3, NiO, CeO.sub.2, HfO.sub.2, Y.sub.2 O.sub.3 or equivalent, as fully described in Japanese Pat. Appln. No. 17877/77 filed on Sept. 2, 1977. According to said Japanese patent application, a catalytic combustion type gas detector based on this technical idea may be made by choosing the special metal oxides as a catalyst. However, it has been found that this gas detector, utilizing the catalytic action of the abovesaid sintered metal oxide, although exhibiting high resistance to catalytic poison and very little degradation in catalytic activity in a high concentration of flammable gas, is unfortunately too sensitive to alcohol vapors and to tobacco fumes, more so than to other gases and vapors. For example, in the case of a gas detector using a sintered ternary metal oxide of TiO.sub.2 --ZrO.sub.2 --Cr.sub.2 O.sub.3, letting the sensitivity to i-C.sub.4 H.sub.10 at a concentration of 2,000 ppm be represented by 1, the sensitivity to C.sub.2 H.sub.5 OH at the same concentration is about 2.9. Accordingly, when employed as LPG (liquified petroleum gas) sensor for home use, such a gas detector would raise an erroneous alarm in answer to alcohol vapors given off by, for example, cooking with wines or spirits and hence is not fully reliable as a gas sensor. As will be described later on, the sensitivity ratio of tobacco fumes to i-C.sub.4 H.sub.10 vapors is about 1.2, that is, the abovesaid gas detector has a higher sensitivity to the former, so that it is likely to given an erroneous warning, for example, when tobacco fumes exist at sufficiently high concentrations.